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The lower Eocene Simsboro formation of the Wilcox group, between the Trinity and Brazos Rivers, Texas, was deposited as a braided stream-floodplain complex. It consists of: (a) very fine-grained to medium-grained sand: immature, clay pellet-bearing subgraywacke bordering on an orthoquartzite; (2) illitic-kaolinitic, silty clay; (3) fine sandstone: siliceous, bimodally mature orthoquartzite; and (4) kaolinitic, silty clay.
The fine- to medium-grained, kaolinitic, clay pellet-bearing subgraywacke occurs as channel deposits and exhibits pronounced trough or festoon cross-bedding. The round kaolinitic clay pellets are detrital. The matrix consists of particles of kaolinite worn from the clay pellets and pellets mashed by the harder detrital grains. Thinly laminated (5-20 mm.) silty clay, in lenticular beds up to 20 feet thick, is laterally associated with the festoon cross-bedded channel deposits. Angular pebbles, cobbles, and boulders of this clay, derived from the nearby flood-plain, are locally incorporated within these channel sands. Hard, siliceous orthoquartzite, consisting of fine-grained quartz in a matrix of quartz silt, forms a massive ledge 2-20 feet thick throughout the area. The exact relation of the orthoquartzite to the rest of the Simsboro is not known. A persistent bed of thinly (1 mm.) laminated, white, kaolinitic, silty clay 20 feet thick occurs at the top of the formation.
Orthoclase and microcline grains severely weathered
to kaolinite and books of authigenic kaolin are common in the subgraywacke and kaolinitic silty clay. Overgrowths of authigenic quartz are present and act as a cement in the orthoquartzite. Chert is rare in the orthoquartzite but is common in the subgraywacke.
The non-opaque heavy mineral suite consists of tourmaline, zircon, and rutile with a smaller amount of kyanite, staurolite, garnet, and euhedral biotite.
The Eocene sediments of the Texas Gulf Coast were derived from multiple sources and the Simsboro is no exception. Quartz types, chert varieties, K-feldspar, phyllitic rock fragments, and heavy minerals indicate that older sedimentary rocks, low-rank metamorphic rocks, granitic or gneissic rocks, and volcanic ejecta furnished detritus for the Simsboro.
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